Final answer:
Fossil fuels are the major storage reservoir of carbon that is not represented in the diagram.
Step-by-step explanation:
The major storage reservoir of carbon, stored in the form of carbon dioxide, that is not represented in the diagram is fossil fuels. Fossil fuels, such as coal, oil, and natural gas, are formed from the remains of ancient plants and animals over millions of years. They are considered a non-renewable resource and are a significant source of carbon emissions when burned.